Laos plans to start building a dam on the Mekong River this week, a step that neighboring Cambodia and Vietnam fear could imperil fish and cripple rice fields.

The Xayaburi dam is expected to reap profits for Laos by generating hydropower to sell to Thailand. The $3-billion-plus project would be the first major dam on the lower part of the Mekong, a massive waterway that provides food and jobs to millions of people in Southeast Asia.

Building the dam marks the latest push toward development for Laos, a growing economy seeking to shed its “least developed country” label. The communist country has opened up its economy over the last few decades and was recently accepted into the World Trade Organization.

“Hydropower is a big natural advantage for Laos,” Deputy Minister of Energy and Mining Viraphonh Viravong told Bloomberg on Monday.

But Vietnam and Cambodia worry dams could endanger fish that families eat and hurt crops in the Mekong Delta, the “rice bowl” of the region. Environmental experts warn if fish dwindle and croplands are lost, damming the river could cost more than it brings in. A Portland State University study last year estimated losses could run as high as $274 billion if a passel of planned dams are built.

A regional report recommended waiting for more studies to assess the dangers, but Laos has pressed ahead.

“They are playing roulette with the Mekong River,” said Ame Trandem, Southeast Asia program director for International Rivers. “There are serious risks for the river and the people who depend on it.”

Laos says it addressed those concerns in the design of its dam, using special features that allow migrating fish to pass through. But Trandem and other environmentalists say those strategies are unlikely to work in the Mekong, populated by some of the biggest fish in the world.

Under an agreement struck 17 years ago, Laos is supposed to consult with Vietnam, Cambodia and Thailand on damming the whole river, under the shared Mekong River Commission. The Laotian government has reportedly argued the agreement doesn’t apply in this case. The decision to build the Xayaburi dam appears to flout the pact.

There was no immediate reaction from Cambodia or Vietnam when Laos announced its plans Monday. Thailand, which is hungry for new sources of power, has agreed to buy the electricity, though a group of Thai villagers are suing their government over the plans and protested on the river Monday.

The news went out the same day that a meeting of European and Asian leaders opened in Laos, a surprise to Southeast Asia watchers who had expected the issue to be hashed out during the summit this week. The way the region reacts could spell out the future of the Mekong.

“This is the opening of Pandora’s box,” said Nathanial Matthews, a hydropower researcher at King’s College London. “Unless there is real pushback in the next six months, I see Laos moving ahead quite quickly with the rest of the dams.”

Eleven dams are planned for the lower Mekong. The next test will be whether Laos follows the same path in pursuing the next project, or whether it bends to neighbors’ concerns. One expert said Xayaburi might have been a unique case because lots of money had already been sunk into preparing the site.

“They could still do it right for the next one,” consulting neighbors and paying attention to their worries, said Richard P. Cronin, director of the Southeast Asia Program at the Stimson Center. “But if they build all of these dams, that will be the coup de grace for the Mekong Delta.”